The invention relates to optoelectronic devices, and, more particularly, to integrated optoelectronic transmitters and receivers and methods of fabrication.
Optical communication systems have advantages over all electrical communications systems with regard to bandwidth, noise immunity, electromagnetic susceptibility, size, and weight are being extensively developed. An optical source converts input electrical information signals into amplitude modulated light for transmission over an optical communications channel, and an optical detector reconverts the amplitude modulated light into electrical signals for reception. Optical communication systems typically employ semiconductor laser sources, glass optical fiber communication channels, and various modulators and detectors. Optical wavelengths on the order of 1 .mu.m permit use of GaAs-Al.sub.x Ga.sub.1-x As diode lasers and silicon photodetectors. FIG. 1a heuristically illustrates a one-way communication system with continuous wave laser diode source 102 modulated by Mach-Zehnder interferometer 104 which incorporates phase modulator 106, optical fiber communication channel 108, and photodetector 110. For amplitude-modulated (analog) light signals, continuous wave lasers with following light amplitude modulators, such as the Mach-Zehnder interferometer, are preferred over just directly modulating the input power of the laser because the laser and the modulator may be separately optimized. A full duplex communication system would have a duplicate of the system on FIG. 1a for communication in the opposite direction. Single-optical-fiber two-way communication could use separate optical wavelengths for the two directions, but this would require wavelength filters. Alternatively, the optical fiber could be split, as shown by splitter 112 in FIG. 1b, but this results in a 50% loss in light intensity to detector 110.
Light modulators may be made from numerous materials in various structures and have various modulation effects. In particular, reflection and phase modulators vary the index of refraction of material in a light path, and absorption modulators vary the absorptance of material in the path. Materials such as ferroelectrics, organic polymers, and semiconductors have been used in modulators. Index of refraction modulation can be had from the Pockels effect, plasma effect, band-filling effect, quantum confined Stark effect, magneto-optic effect, and acousto-optic effect; absorption modulation can be had from the Franz-Keldysh effect, quantum confined Stark effect, and Wannier-Stark localization. For example, Weiner et al., Quadratic Electro-optic Effect due to the Quantum-confined Stark Effect in Quantum Wells, 50 Appl.Phys.Lett. 842 (1987). compute change of the index of refraction of a quantum well with a change of applied electric field based on experimentally measured change of absorption. The index of refraction and the absorption are related (Kramers-Kronig relations) due to the causality of electric field induced dielectric polarization. This allows computation of chirp in a quantum well-based absorption modulator. FIGS. 2a-b show quantum well absorption for applied electric fields of 0 and 65000 volts/cm together with the change in index of refraction for perpendicular and parallel polarized light, respectively, as a function of incident photon energy. Note that large absorption changes enable absorption modulator operation and large index or refraction changes enable electro-optic devices such as directional couplers and modulators. Further increases in the electric field across the quantum well spreads out and shifts the absorption peak as illustrated in FIGS. 2c-d, taken from Weiner et al., Strong Polarization-sensitive Electroabsorption in GaAs/AlGaAs Quantum Well Waveguides, 47 Appl. Phys. Lett. 1148 (1985).
Semiconductor lasers in the form of heterojunction diodes with quantum well active regions and made of materials such as Al.sub.x Ga.sub.1-x As with GaAs quantum wells provide a compact and rugged source of infrared light which can be easily modulated by simply varying the diode current. In particular, a stripe geometry diode laser may be small (e.g., 5l .mu.m wide by 100 .mu.m long with a 30 nm thick active area imbedded in a 400 nm thick optical core). The reflecting ends of the lasing cavity may be distributed Bragg reflectors to avoid cleaved mirror ends. See for example, Tiberio et al., Facetless Bragg Reflector Surface-emitting AlGaAs/GaAs Lasers Fabricated by Electron-beam Lithography and Chemically Assisted Ion-beam Etching, 9 J.Vac.Sci.Tech. B 2842 (1991).